Optical transmission systems have recently been brought into practical use as communications systems. In particular, WDM optical transmission systems are being widely used in optical communications in order to achieve high-speed, high-capacity communications. In the future, with the increasing capacity and speed of communications lines, it is desirable to develop systems that cope with such increases.
In WDM optical transmission systems, to increase the transmission capacity, realization of a large capacity is sought by 1) increasing the transmission speed, 2) decreasing the wavelength intervals of optical signals, 3) applying a transmission speed adaptable for the capacity of data to be transmitted, and 4) applying a modulation format adaptable for the capacity of data to be transmitted. In order to meet these four elements for increasing the transmission capacity, pre-emphasis control may be performed that results in improvement in transmission characteristics in WDM optical transmission systems. The pre-emphasis control is control that adjusts the optical transmission power of multiplexed signals at each wavelength at the transmitter side to improve transmission characteristics at the receiver side.
In optical transmission systems, optical signals transmitted at the transmitter side need to be properly received at the receiver side; however, factors hampering this proper reception related to the use of optical fibers occur. Examples of the factors include wavelength dispersion and nonlinear effects. To properly receive a signal at the receiver side, the Q value corresponding to the signal-to-noise ratio of the received signal needs to be equal to or greater than a predetermined value. However, wavelength dispersion or nonlinear effects may produce degradation in the received waveform, which causes the Q value to be equal to or less than the predetermined value.
There is a technique in which, when pre-emphasis control is performed, it is determined whether an optical signal at each wavelength is in a nonlinear region or in a linear region, and, based on the determination, the pre-emphasis is performed in order to ensure the Q value. This enables pre-emphasis to be automatically performed in a system that transmits and receives wavelength division multiplexed optical signals and in which multiple wavelength channels are multiplexed.
The pre-emphasis control for improving transmission characteristics in a WDM optical transmission system is currently subject to limitations caused by various factors described below, and thus control is becoming difficult.
a) As the wavelength intervals of optical signals are decreased, cross phase modulation (XPM), a nonlinear phenomenon, occurs on a transmission path, resulting in degradation in transmission characteristics.
b) The light power intensity of signals is high, and therefore self phase modulation (SPM), a nonlinear phenomenon, occurs on a transmission path, resulting in degradation in transmission characteristics. Typically, with an increase in the speed of an optical signal, light intensity increases and marked degradation appears.
c) Further, the variety of modulation methods is increasing. In addition to existing on-off keying (OOK), methods such as return-to-zero differential phase-shift keying (RZ-DPSK) and return-to-zero quadrature-phase-shift-keying (RZ-QPSK) are in practical use. Signals having a wide variety of light-intensity characteristics and wavelength interval characteristics are contained together in one system.
Moreover, the transmission characteristics include parameters, such as the Q value and a bit error rate (BER) after transmission, indicating transmission quality in an optical transmission system, and the transmission distance, the intervals at which optical amplifier repeaters are placed, and characteristics of an optical fiber transmission path are determined in accordance with the characteristics of devices, the maintenance policy, and so on. The bit error rate and the Q value correspond to each other at a ratio of 1 to 1, and therefore the inclusion of a BER/Q value conversion table enables a conversion unit 1001 to convert a BER into the Q value. With these system elements under the same conditions, the transmission quality tends to be degraded as the number of wavelengths increases and the wavelength intervals of optical signals decrease. It is therefore important to adjust the output intensities of optical signals in a certain range in order to ensure the transmission quality.
In conventional optical transmission systems, typically, for optimization of pre-emphasis of the transmission output levels of optical signals, the light output levels at each wavelength are increased or decreased within a certain range, so that transmission characteristics are improved. With such conventional methods, transmission characteristics at each individual wavelength can be improved. However, the marked influence on signals at wavelengths adjacent to the increased or decreased wavelength is apparent for a transmission path with steep nonlinear events. It has been impossible to control the Q values at the receiver side of a plurality of wavelengths such that the Q values are within a predetermined range.
Business related to wavelength addition has recently been increasing. In the case of adding a new wavelength to an old system where a transmission path has been constructed, wavelengths are sometimes added such that the number of the wavelengths exceeds the permissible limits of the transmission path designed at the time of system introduction. In such a case, nonlinear events are likely to occur, and it is necessary to add wavelengths while mitigating the effects on the existing wavelengths in operation. This makes it very difficult to optimize pre-emphasis for a plurality of wavelengths.
In more recent systems, there is a larger variety of modulation formats, the transmission speed at each wavelength has been increased, and wavelength intervals between signals have been decreased. In a conventional pre-emphasis method, degradation in transmission characteristics caused by interference of wavelengths adjacent to each other cannot be taken into consideration. There is a demand for a new control method.